In certain vehicles, the internal combustion engine is connected to a transmission, from which driving force is delivered to a transfer mechanism, and the transfer mechanism then feeds and distributes the driving force to the front and rear wheels. In such vehicles, a drive shifting device is associated with the transfer mechanism and interrupts the driving force which is to be fed to either the front or rear wheels, thereby providing shifting between two wheel and four wheel drive.
The above type of vehicle is illustrated in FIG. 9 wherein 302 denotes the vehicle, 304 the front wheels, and 306 the rear wheels. The vehicle 302 has an internal combustion engine 308 disposed therein and connected to a transmission 310. The transmission 310 is linked to a transfer mechanism 312.
The transfer mechanism 312 causes the driving force received from the engine 308 through the transmission 310 to be fed and distributed to a front wheel drive shaft 314 and a rear wheel drive shaft 316. A front wheel differential mechanism 318 and a rear wheel differential mechanism 320 permit the distributed driving force to be transmitted respectively to the front and rear wheels 304 and 306 through front wheel axles 322 and rear wheel axles 324.
The vehicle 302 is provided with a two/four wheel drive shifting device 326. This device 326 has a two/four wheel drive shifting mechanism 328 disposed in the transfer mechanism 312. This mechanism 328 has a transfer shift lever 330. The lever 330 is shifted to interrupt the driving force which is to be delivered to either the front wheels 304 or the rear wheels 306, thereby providing a shift between the two wheel drive and the four wheel drive configurations of the drive train. The mechanism 328 illustrated in FIG. 9 interrupts the driving force to be fed through the transfer 312 to the front wheels 304, thereby providing shift between two wheel drive and four wheel drive.
Further, the two/four wheel drive shifting device 326 is provided with free wheeling mechanisms 332 in order to reduce running resistance which occurs, when the two/four wheel drive shifting mechanism 328 shifts to two wheel drive, because the drive shaft 314 and the differential mechanism 318 are rotated, while both of which are in a non-driven state at the two wheel drive stage. The free wheel mechanisms 332 as illustrated in FIG. 9 are disposed in the right and left front wheels 304.
Upon shifting to two wheel drive, the free wheel mechanisms 332 are brought into a free state so as to drivingly disengage both of the front wheels 304 from the front wheel axles 322. As a result, the drive shaft 314 and the differential mechanism 318 are rotated independent of the front wheels 304, and the latter are thus prevented from producing unnecessary running resistance.
In addition, upon shifting to four wheel drive, the free wheel mechanisms 332 are adjusted into a locked state in order to connect both of the front wheels 304 to the front axles 322. As a result, the driving force to be transmitted through the drive shaft 314 is transmitted to the front wheels 304 through the differential mechanism 318 and the front axles 322.
An example of the above two/four wheel drive shifting device is disclosed in published Japanese Patent Application No. 4-66731. The device according to this publication has a transfer clutch disposed between the engine and one of the respective differential mechanisms for the front and rear wheels, and further has a free wheel clutch positioned between one of the above differential mechanisms and the wheels connected thereto. In this two/four wheel drive shifting device, the free wheel clutch is moved toward a locked position on the basis of a detection signal produced by a sensor. The sensor detects displacement of the transfer clutch toward the locked position. In this way, shifting from two wheel drive to four wheel drive is provided. Then, the two/four wheel drive shifting device is characterized in that, in order to lock the free wheel clutch before a sleeve for disconnecting the transfer clutch is engaged with a clutch-driven gear, the sensor is positioned in such a manner so as to detect displacement of the sleeve toward the locked position and then generate the detection signal before the same sleeve reaches a position where the sleeve initiates the engagement with the clutch-driven gear.
However, with conventional types of two/four wheel drive shifting devices, a problem lies in that a vehicle must be temporarily stopped when shifting between two wheel drive and four wheel drive. In addition, for manually operated free wheel mechanisms, the vehicle's driver must get out of the vehicle in order to switch the free wheel mechanisms between a free state and a locked state. Further, even for automatically operated free wheel mechanisms, when shifting from four wheel drive to two wheel drive occurs, the vehicle must be moved backward in order to switch the free wheel mechanisms from the locked state to the free state.
In such conventional devices, the automatically operated free wheel mechanism typically lacks any means for detecting the free and locked states thereof because of limited space availability. Only desired shifting between two wheel drive and four wheel drive is detected, and an indicating means shows success in shifting to the four wheel drive state.
As a result, another problem arises with the aforesaid two/four wheel drive shifting device which is free of means for detecting the free and locked states of the free wheel mechanisms. That is, when the aforesaid device is operated to shift to four wheel drive, it is impossible to notify the vehicle's driver as to whether or not the free wheel mechanisms are switched to the locked state so as to indicate that shifting to the four wheel drive has actually occurred. A still further problem is that, even when the free wheel mechanisms fail to switch to the locked state as a result of some cause, the indicating means erroneously shows that the four wheel drive is now in operation.
A yet further problem with conventional types of two/four wheel drive shifting device is that, when the means for detecting the free and locked states of the free wheel mechanisms are provided directly in the free wheel mechanisms, there occurs significant increases in component quantity, cost, and man hour as well as constraints on layout due to limited space availability. This is because such countermeasure adds a dedicated detection means to the two/four wheel drive shifting device.